ABSTRACT In Thailand, gay, bisexual, and other men who have sex with men (MSM) account for 41% of new HIV infections with an increasing trend in infections over time. Young MSM (YMSM) are at a significantly greater risk of HIV infection than older MSM with YMSM, ages 15-21, experiencing a 12.1% rate of new HIV infections. There is an urgent need to implement novel behavioral and biomedical interventions for YMSM in Thailand that are culturally and developmentally appropriate, engaging for youth, and scalable. Pre-exposure prophylaxis (PrEP) provides a strong preventative benefit to youth at-risk for HIV infection and there is overwhelming evidence supporting its efficacy. While daily, oral PrEP adherence is highly correlated with its efficacy in clinical trials, adherence rates are variable. In real-world practice settings, PrEP adherence has been found to be lower among youth. Given that YMSM are disproportionately affected by HIV in Thailand, interventions are needed that improve adherence to oral PrEP among this population, thereby maximizing its preventative benefits. To address this need, we propose a collaboration between Duke University, the Thai Red Cross- AIDS Research Centre (TRCARC) and the University of North Carolina- Chapel Hill (UNC-CH). The research team will adapt a theory-based smartphone app to improve PrEP adherence among YMSM in Thailand and conduct a pilot study to assess study feasibility, acceptability, and potential impact. The adapted smartphone app, CHARGE, will utilize social networking and game-based mechanics as well as evidenced-based features to improve PrEP adherence among Thai YMSM, 16-24. In order to adapt the app, we will conduct formative research in Thailand including focus group and Youth Advisory Board discussions with YMSM and interviews with key informants who provide services to YMSM to ensure the app addresses individual, contextual, and cultural factors related to PrEP adherence. Usability testing will assess users? comprehension of the educational content, understanding and use of intervention features, and overall impressions of app relevance and appeal. After any needed app revisions, a 6-month pilot study will be conducted with 60 YMSM initiating PrEP. Participants will be randomized into the CHARGE intervention arm or PrEP standard of care consistent with TRCARC guidelines. The primary outcomes will include study feasibility and acceptability. The secondary outcomes will include PrEP adherence as measured by intracellular levels of tenofovir-diphosphate and emtricitabine triphosphate from dried blood spots. Built on a successful, evidence-based platform designed by our collaborating technology partner, Ayogo, and tested by the study team, CHARGE will be flexible and responsive to changes in technology and changes in PrEP practice standards and guideline recommendations. We have proposed a strong research capacity building plan that will enhance the scientific and technological capabilities of TRCARC and other community based organizations in Thailand to develop, implement, and evaluate mHealth interventions.